1. Technical Field
The present invention relates in general to enhancing the duty cycle of a thermoelectric cooler element, and in particular to a method and system for achieving greater efficiency from a thermoelectric cooler. Still more particularly, the present invention relates to a method and system of thermally switching a thermoelectric element such that reduced thermal resistance can be attained from a thermoelectric element to a hot source during intermittent periods within a heat transfer cycle of a thermoelectric cooling process.
2. Description of the Related Art
The utilization of thermoelectric devices in industry has, to date, been restricted to very specialized applications. Due to inefficiencies, very few applications can effectively utilize thermoelectric effects. The undesirable properties of thermoelectric elements such as high cost and low efficiency are outweighed by the desirable properties of thermoelectric devices. Recently, there have been significant advances in material technology, many attributable to advances made by the semiconductor industry.
Conventional cooling systems, such as a those found in a refrigerator, utilize vapor compression refrigeration cycles to provide heat transfer. Vapor compression cooling requires significant moving hardware, including at a minimum, a compressor, a condenser, an evaporator, and related coolant transfer plumbing. Miniature vapor compression cooling is not available for small cooling applications.
Semiconductors and superconductors have enhanced performance at lower temperatures. CMOS logic can operate materially faster at lower temperatures. For example, if CMOS logic devices are operated at -50.degree. C., performance is improved by 50 percent over ambient room temperature. Liquid nitrogen cooling of CMOS logic to -196.degree. C., has shown a 200 percent performance improvement in speed.
Similar benefits have been discovered for integrated circuit wiring. Wiring resistances decrease by a factor of two for integrated circuits operated at -50.degree. C. in comparison to room ambient temperature operation.
Thus, sub-ambient temperature operation of integrated circuit logic devices, such as field effect transistors, as well as the interconnect wiring can materially improve integrated circuit performance. However, accomplishing such cooling in the confines of an ever decreasing size poses new challenges.
The dimension of integrated circuit transistors is continually decreasing and the density of transistors is ever increasing. Faster switching speeds or more switching transitions per unit time also contributes to additional heating. Currently, switching speeds of over one gigahertz are being implemented and adequate cooling has increasing importance in such devices.
Analog circuits, such as voltage controlled oscillators, phase detectors, mixers, and low noise amplifiers produce more heat than digital circuits. Additionally, lasers and photo diodes have remarkably improved performance and resolution at lower temperatures. Thus, cooling of sensors within mass data storage devices is increasingly important. Hot spots within an integrated circuit can cause a host of related failures. As the dimension of integrated circuits decrease and become more compact, the dissipation of internally generated heat becomes an increasing problem.
Thermoelectric cooling is one alternative that has found some utilization given the compact size of Peltier devices. Peltier device thermoelectric cooling is very reliable because such devices are solid state. The inefficiency of thermoelectric devices is a key negative aspect of implementing a thermoelectric cooling design. A Peltier device cooling system typically has an efficiency in the range of 20 percent for a relatively nominal temperature differential between the cold sink and ambient temperature conditions.
For example, utilizing a Peltier cooling system to cool at a rate of one watt and attain a sub-ambient temperature of 0.degree. C. requires that the system be powered with five watts. As the amount of heat to be transferred increases, the total power to be dissipated into the environment mandates large convection devices and large power supply circuits. Therefore, Peltier device thermoelectric cooling has not been considered a broadly applicable technology for cooling electronic and optical devices.
Typically, Peltier devices are fabricated from semiconductor material such as bismuth telluride or lead telluride. Commonly utilized, Peltier materials exhibit very high electrical conductivity and relatively low thermal conductivity. In contrast, most metals have both high electrical and high thermal conductivity.
In operation, a Peltier device transports electrons from a cold sink at temperature T.sub.cold to a hot source at temperature T.sub.hot in response to an electric field placed across the Peltier device.
FIG. 1 schematically depicts a conventional Peltier type thermoelectric element (TE) 1 with DC power supply 2 creating an electric field across TE 1 and a load current 3. The desired heat transfer is from cold mass 4 at temperature T.sub.cold to heat exchanger 6 at temperature T.sub.hot. The basic heat transfer of a thermoelectric element is represented below. EQU q=.alpha.T.sub.cold I-1/2I.sup.2 R-K.DELTA.T Equation 1:
The net heat energy transported by a Peltier device is composed of three elements. In equation 1, the first element a T.sub.cold I represents the Peltier effect (thermoelectric) contribution, the second element 1/2 I.sup.2 R defines negative Joule heating or resistive effects, and the third element KAT defines negative conductivity effects (back flow) of the heat. The thermoelectric component is composed of the Seebeck coefficient .alpha., the temperature of operation (T.sub.cold) and the current through the TE device I.
Approximately one half of the Joule heating produced by the bias current is conducted to the cold sink and the remainder to the hot source. Lastly, the negative component attributable to thermal conduction represents the heat flow or heat conduction through the Peltier device. K is the thermal conductivity of the Peltier device from the hot source to the cold sink.
In equation 1, the thermoelectric component of the heat transport increases linearly with the current through the Peltier device and the Joule heating increases in proportion to the square of the current. Alternately described, the resistive heating exponentially increases due to the current through the Peltier device while the cooling effect linearly increases with increased current flow. The thermal conduction is also in direct proportion to the temperature differential between the hot source and the cold sink. Equation 1 clearly reflects how quickly the Peltier device becomes inefficient.
Equation 2 below defines a coefficient of performance for a Peltier device. The coefficient of performance is the ratio of the net heat energy transported at low temperature to the power consumed in the Peltier device. For a typical Peltier device made from bismuth telluride material, the coefficient of performance is less than 0.3 for a temperature differential of 30.degree. K. ##EQU1##
Note that the numerator of equation 2 represents the net cooling capability of the Peltier device. The denominator of equation 2 represents the total energy provided by external DC power supply 2. The individual elements of the numerator are described above in reference to equation 1. The first element in the denominator is the total Joule heating, while the second element is the heat energy transport work done by the Peltier device in moving energy from the T.sub.cold sink to the T.sub.hot source. Based upon this relationship, the maximum coefficient of performance possible in the configuration of FIG. 1 is given by equation 3. ##EQU2##
The parameter .gamma. can be expressed in terms of the Seebeck coefficient .alpha., electrical conductivity .sigma. and thermal conductivity .lambda. as set forth in equation 4. ##EQU3##
The first factor in equation 3 T.sub.cold /.DELTA.T is the maximum efficiency possible for any heat pump operating between two thermal sinks T.sub.cold and T.sub.hot. T.sub.cold /.DELTA.T is commonly referred to as the Carnot efficiency. The second factor represents the non-ideal thermoelectric cooling, which can also be characterized by a figure of merit ZT. Note that .eta..fwdarw. (T.sub.cold /.DELTA.T) as .gamma..cuberoot..infin.. To date it has been very difficult to develop a thermoelectric material which yields high values of ZT.
Historically, the prevalent material for thermoelectric coolers has been bismuth telluride (Bi.sub.2 Te.sub.3) and lead tellurium (PbTe). These materials have ZT values of approximately 0.3 at room temperature. Recent research has shown that ZT values approaching one may be possible in lead tellurium quantum wells and multi-lattices. However, even with these materials, thermoelectric devices have not produced practical cooling solutions.
Another constraint of Peltier device cooling is that only a limited temperature excursion below ambient temperature is attainable. The temperature differential limitation arises from the fact that the effective temperature differential is constrained by efficiency. Efficiency degrades quickly with an increasing temperature differential between a hot source and a cold sink. The maximum temperature differential possible T.sub.max is given by equation 5 below. EQU .DELTA.T.sub.max =1/2ZT.sup.2.sub.cold Equation 5
For bismuth telluride having a ZT of approximately 0.3, T.sub.max is 45.degree. K. at 300.degree. K., where 32.degree. F. is equivalent to 273.degree. K.
Thus, there are a number of very fundamental constraints on efficiency and differential temperature that limit the practical utilization of conventional thermoelectric elements for cooling applications. Particularly, applications which utilize ambient temperatures to dissipate the heat are impracticable. Typically, each Peltier device is small in dimension and can only transport a finite amount of heat. Therefore, to produce a cooling effect of desired magnitude many Peltier devices must be connected together.
Enhancement to basic thermoelectric cooling designs can be achieved by switching the thermoelectric elements. Peltier cooling is fast (sub-microsecond) while thermal conduction is slower (millisecond). The difference in temporal scales can result in enhanced cooling under transient conditions. Thermal and electrical switching can be accomplished by microelectromechanical switches and by tunnel junction heat switches. Switched coolers increase the efficiency of Peltier devices and permit maximum temperature differentials as large as 200.degree. K.
It should therefore be apparent that there is a need for method and system for selectively powering and selectively thermally switching thermoelectric devices to enhance the efficiency of a thermoelectric cooler.